Two distinct types of junctions, chemically transmitting synapses and electrotonic junctions, will be studied in order to further elucidate the morphological basis of neuro-neuronal and neuro-muscular transmission. Studies on the fine structure of chemical synapses in Mauthner fiber-giant fiber junctions of hatchetfish and in motor endplates of frog and rat under different controlled physiological conditions to test and extend the vesicle hypothesis for synaptic transmission will be carried out. The fine structural and permeability properties of rectifying electrotonic junctions (the lateral giant-motor fiber synapse of the crayfish and the giant fiber-motoneuron synapse of the hatchetfish) will be compared with those of other electrotonic junctions. In addition, our finding of gap junctions between normally differentiating skeletal muscle cells of Ambystoma embryos will be studied. Experiments are planned to determine whether the relationship of the gap junctions between adjacent myotomes and the formation of mature endplates play a role in the development of the axial musculature.